Symbiotic organisms must use a variety of mechanisms to maintain their relationship. Often, they produce signals that their partner specifically responds to, inducing physiological changes that facilitate development or maintenance of the symbiosis. The relationship between the gamma proteobacteria Vibrio fischeri and the Hawaiian bobtail squid Euprymna scolopes is a well-studied mutualism. V. fischeri is the only bacterium capable of colonizing the squid's light organ immediately after hatching, and continues to reside within the crypts of the light organ throughout the life of the host. In exchange for various nutrients and a stable environment, V. fischeri produces light at high densities, which is thought to mask the squid's shadow while it is nocturnally active. V. fischeri produces many molecular signals that induce physical changes in the squid light organ, such as lipopolysaccharide and tracheal cytotoxin (TCT). Recently, we found that, like all other gram-negative bacteria and many gram- positive bacteria, V. fischeri produces outer membrane vesicles (OMV). OMV often carry specific molecular cargo between bacteria and other organisms, mediating their interactions. Interestingly, these OMV were able to provoke a basic response from the squid that mimicked that of the entire bacterium. Also, this response was orthogonal to that induced by other known stimulants, indicating that OMV represented a novel means of communication between host and bacterium. This project aims to discern the composition of V. fischeri OMV, and to measure how these components affect the squid-Vibrio symbiosis. We will also assess the importance of OMV as a delivery mechanism, and predict the importance that these objects have on other mutualistic relationships.